Due to less energy consumption and less environmental impact, as well as more sugar yield, methods of hydrolyzing biomass by using cellulase have been widely studied. However, there is a drawback in such a method of hydrolyzing biomass by using cellulase in that the cost of sugar liquid production increases due to the high price of cellulase. To solve such a technological problem, methods of recovering to reuse cellulase used in the hydrolysis have been suggested. Nonetheless, the cellulase is strongly adsorbed by hydrolysis residues generated when cellulose-containing biomass is hydrolyzed and therefore enzyme's reusability is low, which has been problematic.
As for methods of desorbing the cellulase adsorbed by the hydrolysis residue to increase the recovery of cellulase, disclosed are a method of washing hydrolysis residues with an aqueous sodium hydroxide solution of about pH 8 (D. E. Otter et al., “Elution of Trichoderma reesei Cellulase from Cellulose by pH Adjustment with Sodium Hydroxide,” Biotechnology Letters (1984), Vol. 6, No. 6, 369-374), a method of bringing hydrolysis residues into contact with a nonionic surfactant (Japanese Patent Application Laid-Open Publication No. S63-87994), and the like. Meanwhile, as for methods of decreasing the adsorption of cellulase to hydrolysis residues, known are a method of adding water-soluble salts at the time of the hydrolysis of cellulose-containing biomass to adjust an electrical conductivity to 5 to 25 mS/cm (Japanese Patent No. 4947223), a method of adding calcium carbonate particles in 1 to 10% by weight based on the solid weight of cellulose-containing biomass (Japanese Patent Application Laid-Open Publication No. 2012-100617) and the like.
As mentioned above, various attempts have been made to reduce cellulase consumption by recovering and reusing the cellulase used for the hydrolysis of cellulose-containing biomass. But, the problem has not been solved because the cellulase is strongly adsorbed by hydrolysis residues and the recovery is thus low.
It could therefore be helpful to provide methods of producing a sugar liquid capable of more effectively reduce the cellulase consumption than conventional methods.